(1) Field of the Invention
This invention relates to proximity sensing apparatus; and, more particularly, to a magnetic proximity sensing device especially adapted for sensing ferrous metal such as the location of a ferrous piston in a fluid motive power cylinder, counting ferrous metal parts, or the proximity of some other ferrous metal member.
(2) Prior Art
It is well known to control various apparatus by sensing the operation of fluid motive power cylinders of either the pneumatic or hydraulic type. By sensing the presence or position of the movable piston within the cylinder, one can control the operation of other apparatus in response to movement of that piston or, alternately, control the length of the stroke of the fluid cylinder piston itself.
Although many types of proximity sensors have been previously devised, many require the presence of magnetic bodies within the part whose movement or proximity is being sensed in order to properly operate the proximity sensor. This is a particular problem with fluid motive power cylinders, a great many of which are manufactured without magnets in their piston bodies. Another problem often encountered is providing a sensor which can be relatively easily manufactured at a relatively low cost, yet continue to provide reliable indication of proximity under extended usage. In particular, sensors with mechanically linked parts tend to be susceptible to wear during extended use and to malfunction due to rough handling.
Known proximity sensors include applying the oscillations of an inductance and capacitance bridge network to a switching network. That is, the introduction of a ferrous material in proximity to a coil, providing the inductance, changes the oscillations in the inductance and capacitance bridge. The switching network includes a silicon controlled rectifier in combination with a diode bridge network. Such a proximity sensor is relatively complex and requires a relatively sensitive balancing of the inductance and capacitance bridge. Further, the switching network has two stages so that the signal from the inductance capacitance bridge is sufficiently amplified to cause switching between a power source and a load. Such a requirement of two stages also adds undesirable complexing and cost to a sensor.
Still another problem typically encountered in the use of proximity sensors with fluid cylinders is the attachment of the sensor to the body of the cylinder. For proper operation it is necessary that the sensing apparatus be secured as closely as possible to the cylinder itself. Prior known structures have included complicated, rigid structures fitting around the body of the cylinder which hold the sensor tightly against the cylinder body. Relatively rigid structures have also been used to attach the sensing apparatus to cylinder tie rods. However, since adjustment of the position of the sensors is often necessary when the piston stroke must be adjusted, such prior structures have often been difficult to move or adjust. Other structures have fixedly mounted the sensing apparatus at the ends of the cylinder eliminating flexibility necessary for complete utilization of the sensing apparatus.
In addition, it is desirable from a manufacturing standpoint to have a proximity sensing apparatus which is adaptable to many uses such as for counting metal parts, sensing the location of moving metal parts on rams or presses, or the like.
The present invention overcomes the above problems by providing a sensing apparatus which substantially eliminates the use of moving mechanical parts and is easily and securely attached to the existing structure of fluid cylinders and other apparatus without the necessity of providing additional complicated, rigid structure and without the necessity of disassembling any portion of the fluid cylinder. The present invention is self contained and does not require the addition or inclusion of magnets on any part of the body being sensed. Thus, the invention is especially useful with pre-existing fluid cylinders which do not include magnets in their piston bodies. Also, the invention can be effectively used in many applications such as sensing metal parts to count or locate the same, during inspection, serving as a limit switch or trigger for another event or various machinery, or providing a sensor in a visible or audible signaling system.